Gas tube surge arrestors are broadband radio frequency (RF) devices that are capable of allowing a DC-bias to pass along the center conductor of a transmission line while harmlessly shunting the lower frequencies associated with faults or transients to ground. Faults or transients may be induced, for example, by lightning. Many gas tube surge arrestors contain a replaceable gas tube element. Under normal operating conditions, the gas tube element functions as a simple capacitor with a shunt capacitance of typically less than 1 picofarad. However, in the presence of a transient, the gas within the gas tube element ionizes and switches the gas tube element to a low impedance state. The gas tube element may be filled with any suitable inert gas. In this low impedance state, the gas tube element harmlessly passes any surge current present on the center conductor of a transmission line to ground.
Gas tube surge arrestors are known in the art. An example of one type of known gas tube surge arrestor is shown in FIG. 1. There, a surge arrestor 110 includes a surge is arrestor cap 112, an annular seal 114, a bent disk spring 102, a retention clip 100, a gas tube element 118, a center disk 120, and a center conductor 122. The retention clip 100 is best shown in FIGS. 2a and 2b. The retention clip 100 fits into the surge arrestor cap 112. The retention clip 100 was used to retain the gas tube element 118 in the surge arrestor cap 112. The gas tube element 118 includes opposing first and second electrodes 140, 142. The electrodes 140, 142 include radial rims which project radially beyond the outer diameter of the body of the gas tube element 118. Other known gas tube elements have electrodes that include radial rims which may not project radially beyond the outer diameter of the body of the gas tube element. In any case, the retention clip 100 snaps over and/or beyond the electrodes and applies inward pressure on the gas tube element 118 to secure the gas tube element 118 in the retention clip 100.
The surge arrestor cap 112 unscrews from the surge arrestor 110 to access the gas tube element 118 housed in the surge arrestor cap 112, as shown in FIGS. 1 and 3. To remove the gas tube element 118 from the retention clip 100, axial pressure was applied in a direction away from the cap 112. However, the retention clip 100 had the dual functions of retaining the gas tube element 118 and securing itself in the cap 112. These two different functions forced designers to produce the prior retention clip 100 and cap 112 within very tight tolerances. Consequently, the cost and time required to produce the retention clip 100 was increased. Likewise, the tight tolerances made installation of the retention clip 100 in the cap 112 and installation of the gas tube element 118 in the retention clip 100 more difficult. In addition, a separate piece, the bent disk spring 102, was provided to apply downward pressure between the gas tube element 118 and the center disk 120 to ensure a good electrical connection between the first electrode 140 and the center disk 120 which was connected to the center conductor 122 of a coaxial geometry. The bent disk spring 102 also provided pressure between the gas tube element 118 and the cap 112 to ensure a good electrical connection between the second electrode 142 and the inner surface of the surge arrestor cap 112. However, inclusion of an additional piece, the bent disk spring 102, made installing the retention clip 100 into the cap 112 more expensive.
The opposing inner surfaces of the electrodes 140, 142 within the gas tube element 118 define an arc gap, as shown in FIG. 5. The path that a transient surge follows is also shown in FIG. 5. For example, if an antenna were struck by lightning, that current surge would be conducted through the antenna, through the center conductor 122 of the connected transmission line, through the gas tube element 118 by sparking between the two end electrodes 140, 142 (as shown in FIG. 5a), through the retention clip 100, through the bent disk spring 102, through the metal surge arrestor cap 112, through the body of the surge arrestor 110 to a metal bulkhead 104 and finally to ground. FIG. 4 shows how the surge arrestor 110 is connected to the grounded metal bulkhead 104.
The drawback of the prior surge arrestor clip design is that it consisted of two individual parts, a retention clip 100 and a bent disk spring 102. This two-piece design increased the difficulty of installing a new gas tube element and increased orientation errors during assembly. In addition, the two pieces would often get caught together during shipping and handling, making them difficult to separate. Moreover, the prior two-piece design was difficult and expensive to manufacture. For example, it takes approximately 400 hours to manufacture 25,000 bent disk springs and 25,000 retention clips. The cost to manufacture 25,000 bent disk springs is approximately $2.00 each and the cost to manufacture 25,000 retention clips is approximately $3.30 each; thus, the two-piece design costs approximately $5.30 each to produce.
Therefore, there is a need for a simpler surge arrestor clip design that reduces the number of parts required, greatly reduces production costs, and reduces the complexity of assembling the surge arrestor and installing and replacing gas tube elements.